csci 465 d ata communications and networks lecture 21 martin van bommel csci 465 data communications...

CSCI 465Data Communications and Networks

Lecture 21

Martin van Bommel

CSCI 465Data Communications & Networks

1

Ethernet

• Developed at PARC – Palo Alto Research Center• Inspired by earlier networks• Named “ethernet” after “ether”– Name used by physicists in 1800s to refer to the

“substance through which electromagnetic energy traveled”

– Thought to be everywhere but invisible– Networks will be universal but invisible


2

Ethernet Precursors

• ALOHA – packet radio network– Station may transmit frame at any time– Station listens for acknowledgement• time > 2 X propagation delay + fixed increment• If none, resend – corrupted due to noise or collision

– Simple, but limited to about 18% efficiency

• Slotted ALOHA– Time divided into organized slots equal to frame– Increases utilization to about 37% efficiency


3

Ethernet Precursors

• Carrier Sense Multiple Access (CSMA)– Before transmit, listen to medium (Carrier Sense)– If medium idle, station transmits– Wait for acknowledgement• Time is longer than ALOHA to account for contention

– If no acknowledgement, listen and retransmit– Utilization far exceeds ALOHA• Longer frames and shorter propagation delay gives best

utilization


4

Non-Persistent CSMA

• Listen to medium• If medium is idle, transmit• If medium is busy, wait an amount of time

randomly drawn from a probability distribution, and retry

• Disadvantage– Channel usually remains idle following

transmission because stations are waiting


5

1-Persistent CSMA

• Rules– If medium idle, transmit– If medium busy, listen until idle, then transmit

immediately

• Selfish strategy– If two or more stations are waiting, collision is

guaranteed


6

P-Persistent CSMA

• Rules– If medium idle, transmit with probability p or

delay one time unit with probability (1-p)– If medium busy, listen until idle and repeat– If transmission is delayed one time unit, repeat

from step 1

• How do we choose effective value of p?– Too high – everyone tries to send– Too low – waiting for nothing


7

CSMA/CD (Ethernet)

• Rules– If medium is idle, transmit– If medium is busy• Continue to listen until idle (1-persistent)• Transmit once idle but keep listening

– If collision detected while transmitting, stop– After collision, wait a random amount of time,

referred to as the “backoff”, then start again


8

9

CSMA/CDOperation

Binary Exponential Backoff• For backoff stability, IEEE 802.3 and Ethernet

use binary exponential backoff• Stations repeatedly resend when they collide– on first 10 attempts, mean random delay doubled– value then remains same for 6 further attempts– after 16 unsuccessful attempts,

station gives up and reports error

• Efficient over a wide range of loads• However, has last-in, first-out effect


10

Collision Detection

• On baseband (co-axial cable) bus– Collision produces higher signal voltage– Detected if cable signal greater than station’s

• On twisted pair (star topology)– Activity on more than one port is collision– Use special collision presence signal from hub

until all incoming signals end


11

802.3 Medium Notation

• Notation format:<data rate in Mbps><signaling method><maximum segment length in 100s of meters>

• e.g 10Base5 provides 10 Mbps baseband, up to 500 meters

• T and F are used in place of segment length for twisted pair and fiber


12

10Base5 “Thick Ethernet”

• Original 802.3 medium specification• 50-Ω coax and Manchester signaling• Segment length can be extended past 500m

with repeaters– transparent at the MAC level– maximum of 4 allowed

• No looping allowed--one path between any two stations


13

10Base2 “Thin Ethernet”

• Intended to provide lower-cost system for PC LANs

• Uses thinner cable and supports fewer taps than 10Base5

• Can combine 10BASE2 and 10BASE5 segments in the same network – but backbone must then be 10BASE5


14

10BaseT

• Uses UTP• Star-shaped topology is well-suited to existing

wires terminating in a closet• Stations attach to central multi-port repeater

(hub) via two twisted pairs• Hubs can be cascaded• Physical star, but logical bus – all transmissions are repeated


15

10Broad36

• Only 802.3 broadband specification• Uses 75-Ω CATV (cable TV quality) coax• Maximum length of individual segment is

1800m• Broadband is by nature analog, so analog

encoding must be used (PSK)


16

10Base-F

• Standard includes 3 specifications– 10-BASE-FP: Passive star topology, up to 1km per segment– 10-BASE-FL: Point-to-point link connecting stations or

repeaters up to 2km– 10-BASE-FB: Point-to-point backbone link connecting

repeaters at up to 2km

• All specs use two fibers– one for transmission in each direction

• Manchester encoding– converted to optical signal elements


17

Ethernet Frame Format


18

Pre SOF Dest Source Len Data CRC

• Preamble - 7 byte pattern of 1’s and 0’s - synchronization• Start of Frame - pattern 10101011• Destination and source addresses - 2 or 6 bytes (12 Hex)• Data length field - 2 byte value (<65536)• Data field - up to 1500 bytes• Pad field - if data field too short, padded up to 46 bytes• Frame check sequence - 32-bit CRC error check (4 bytes)

Ethernet Frame Length

• Why is data + pad b/w 46 and 1500 bytes?• 46 data + 18 control = 64 bytes minimum• Shorter frame length would not allow

sufficient collision detection– Frame must still be in transmission when a

collision of the first bit at other end of network has time to propagate back

• Longer frame length lets one station dominate for too long


19

Ethernet Maximum Distance

• 10Base5 with no more than four repeaters • 5 segments, max 500 m each• max 50 m long transceiver cables to two

stations at the ends• Max distance station to station = 2600 m• 10Base2 (Cheapernet) – max 200 m per segment, no transceivers, gives

1000 m max


20

Remote Repeater

• Connects two segments over distance of up to 1000 m

• Consists of two local repeaters connected by fiber optic link

• Can only have it and one other repeater between any two stations


21

Ethernet Address

• 48 bit integer (281 trillion possibilities)• Assigned to interface card by manufacturer

who purchases set of addresses from IEEE• No two interfaces have same address, thus

changing interface changes address• Higher levels of software must accommodate

address change


22

10Mbps Ethernet Summary

• 10 Mbps broadcast bus technology– all stations share single communication channel– all nodes receive every transmission

• Best-effort delivery semantics– no acknowledgement of receipt at MAC layer

• Distributed access control– CSMA/CD with max 1500 bytes data per frame– Min idle time between transmissions (12 bytes)


23

Ethernet Applications

• Good network for short, bursty traffic• No guaranteed response time• Collisions serious problem when utilization

somewhere between 20% and 70% ???


24

csci 465 d ata communications and networks lecture 21 martin van bommel csci 465 data communications...

Documents

mediumif medium

transmitif medium busy

pif medium busy

idle following transmission

acknowledgement time

time unit

probability p

shorter propagation